Sorry if this has been asked before!

I have just booked a flight and paid using my Amex card. The first time I attempted payment, I mistakenly entered the wrong 4-digit card security code (number on the front). The website said it rejected the payment but my Amex app piped up that the payment was made and it's now "pending" on my Amex account, as is the second payment made with the correct security code, that has led to a confirmed booking.

Any idea why BA does this (create a "pending" payment even when it's been rejected)? If I were close to my credit limit this would have presumably have prevented me from making a second attempt at payment and booking the flight.